First off, this would be an AWESOME! idea if done right, and it would have to be done right for it to work at all. But to add my two cents of what the general plot should be, we first need to establish a few things. First, confirm that these movies all take place in a similar land, similar to the Enchanted Forest from OUAT, by using popular internet theories (allowing for us fans to feel that we have had an impact on both the movie and society). Second, don't jump the gun on using all the princesses at once; instead take a couple from each of the three eras (original, renaissance and revival), with Easter eggs hinting at the others existence. Lastly, do the villain(s) right- make them threatening, but not too overpowered.
Next we determine our cast. Our heroines are going to be Aurora, Cinderella, Belle, Ariel, Rapunzel and Anna. Elsa will have a brief cameo but it will be little more than a use of world building. Our villains are going to be Maleficent and Hans, with Hans having made a sort of deal with the devil for Maleficent's help to enact revenge on Arendelle. Of course, our princely counterparts will be there, but I'll get to them in a moment.
Now, the most important part: the plot. We start on Anna and Kristoff's wedding; our other princesses are there to celebrate the joyous day, and it's here we have our first musical number (yes this is a musical, I guess I should have mentioned that earlier), maybe a reprisal of 'Love's An Open Door' where the words are changed to reflect the sisterly affection of our leads, as well as the excitement for Anna and Kristoff. But as the priest asks if anyone has reason why they shouldn't be wed, Hans bursts in and causes a scene. All the other princes, including Kristoff, are needed to remove Hans from the church as he's been made stronger by Maleficent somehow (maybe a potion she got from Hades). As the princesses comfort Anna at the disruption to her wedding, we see a large shadow pass outside and then an eerie light flashes and Maleficent's evil cackle can be heard. The princesses then rush outside to find the clothes of their princes but the princes are nowhere to be found.
Suddenly from the clothes of Philip, a mouse emerges and runs to Aurora, who's a little frightened by the sight of the mouse. Cinderella, quickly strikes up a conversation with mouse Philip and relays what happened to the others. Hearing that the other princes were all turned to mice and kidnapped by Maleficent who appeared to be working with Hans, cripples the princesses with grief, but also provokes their sense of adventure; they now have an opportunity to save their husbands and prove their mettle in one fell swoop. After a brief argument between Anna and Elsa about Elsa's decision to stay in Arendelle (claiming to ensure nobody attacks while Anna's away, as well as sending out messages for help from foreign lands [China and Agrabah, for example]) the main heroines embark out to save their princes. This is also where we get our second musical number (I'm thinking a mash-up of 'I'll Go The Distance' and 'I'll Make A Man Out Of You'; I know those songs aren't from our main cast's movies but linking the songs to this movie is another form of world building and connecting this new franchise's universe; also how funny would it be to see Cinderella and Aurora singing slightly altered Mulan lyrics to a mouse Philip.
But before our heroines can save their loved ones, they first need to arm up; so they head to Belle's castle to gather as much info on magic as they can find. At her castle, we bump into all our favourite non-titular B&B characters, and a surprise cameo from one of the seven dwarfs (I'm going to say Dopey because the thought of him and Lumiere pulling pranks on Cogsworth is too funny). While Belle and Ariel look for ways to reverse Maleficent's spell (Ariel, as a mermaid, is the only one able to wield magic) and practice with smaller spells, Rapunzel and Anna train Cinderella and Aurora how to fight; Rapunzel insists that they all learn the newest fighting style in Corona: Kampfbratpfannen, the art of the frying pan (of which she's a master), meanwhile Anna teaches a more traditional style-random fist-a-cuffs. As this montages, a reworking of 'Friend Like Me' and 'Friends On The Other Side' is sung by the princesses.
Meanwhile, at the dark castle of our villains, we find Maleficent and Hans arguing about their deal. Hans isn't happy and thinks that they should have taken out Anna and Elsa and not the princes; Arendelle was the target, not the love lives of the princesses. Maleficent at first dismisses him, but once he becomes more insistent she threatens him by transforming him into a mouse before breaking out into her villain song (this should be an original piece as Maleficent deserves her own dark, creepy and bombastic villain song). At the end of the song, Hans is returned to a human, visibly disturbed by the ordeal and song, as he now realizes what he's gotten himself into by making his deal with Maleficent.
Back with our heroines, we find them approaching our villains' lair. As they approach the break out into a reprisal of the version of 'I'll Go The Distance', mixing in some of 'The Mob Song' to add a bit of tension to the final battle. Outside the gates, they divide into three groups: Ariel and Cinderella go to find the mice princes and turn them back to human, Anna and Belle act as distractions by drawing out Hans and a useless battalion of Maleficent's minions, and Aurora and Rapunzel go to find Maleficent and stop her from interfering in the others' missions. Things go slightly awry when it's revealed Maleficent has captured Cinderella and Ariel, and it looks like everything has been for nothing as a haggard Belle and Rapunzel enter the room battling a battered Hans. And just as Maleficent is about to deliver the final blow (after a brief villain monologue "Mistress of All Evil Blah Blah All the Powers of HELL!), a hooded figure crashes through a window and fires an arrow at Maleficent's staff. The staff's orb shatters and the princes are returned to human form and Hans falls to the floor from his injuries. The archer is revealed to be Merida, who got a letter from Elsa and immediately set out knowing the toll having your loved ones turned into animals can have on a princess. Maleficent, weakened by her staff's destruction, threatens her return before turning into a dragon and escaping.
Relieved to have their husbands back, our heroines breakout once again into song (this time a rehash of 'Now That I See You'), we see Merida make a face at all the lovey-doveyness of the song, and the scene dissolves mid-song to where we started-Anna and Kristoff's wedding-and as the song ends they kiss to the cheers of everyone around them. Roll Credits.
To go the full cinematic universe route we have two post-credits scenes. The first is actually mid-credits and shows the back of a female warrior arriving in Arendelle to the shocked looks of everyone (Mulan is finally here). The second comes at the end of the credit and starts off as darkness with eerie music, until we see Maleficent's staff being repaired and returned to her by a figure with its back to us. After she thanks her master for returning the staff, the figure unfurls its bat wings and growls menacingly (Chernabog is our universe's Thanos, for the MCU comparison).